A two-piece paper cup produced by a machine of the type to which this invention relates is made up from a side wall blank and a circular bottom wall blank. If the cup is to be cylindrical the side wall blank is rectangular. For a downwardly tapering cup the side wall blank is more or less keystone-shaped with arcuate top and bottom edges and straight, upwardly divergent side edges. In either case the circular blank that is to form the bottom wall of the cup has its marginal portion deformed into a downwardly projecting flange.
Insofar as the mechanisms for manufacturing tapered cups differ from those for producing cylindrical cups, tapered cups present somewhat more complex problems. Therefore, for purposes of example, the following explanation is specific to the manufacture of tapered cups but it is also generally applicable to untapered cylindrical paper cups and containers.
The machine on which the blanks are assembled and formed into a cup has a turret with a vertical axis, comprising a number of mandrels that project radially from the axis and are carried to each of a succession of stations by indexing rotation of the turret. In a machine for making tapered cups the mandrels are frustoconical, and each tapers in the direction away from the turret axis and has at is small diameter outer end a flat surface in which there are suction inlet openings. In the operation of the machine, a flanged bottom wall blank is concentrically attached by suction to the flat end surface of a mandrel, with its flange projecting away from the mandrel. Thereafter a side wall blank is wrapped around the mandrel and the bottom wall blank. This wrapping brings the marginal side edge portions of the side wall blank into an overlapped relationship in which they can be bonded to one another to form a side seam. It is also necessary for the flange of the bottom wall blank to be bonded to the side wall to form a bottom seam. Other operations may then be performed upon the top and bottom edges of the cup side wall, but the formation of well bonded, leak-proof side and bottom seams is of the essence.
U.S. Pat. No. 3,134,307 to D. B. Loeser disclosed apparatus for forming the side seam of a cup, comprising a plurality of heated elements on the turret, one adjacent to each mandrel. When a side wall blank was wrapped around a mandrel, the heated element adjacent to that mandrel moved down into engagement with the overlapped side edge portions of the blank, clamping them against the mandrel. The heated element remained in contact with the blank through a certain amount of turret rotation, melting the plastic coating on the portions of the blank that it engaged, and then it moved away from the mandrel to be replaced by a cooling element which similarly engaged the side wall seam through further stages of turret rotation to cool and solidify the molten plastic into an adhesive bond. The apparatus was complicated and somewhat troublesome because it had to have a heating element and a cooling element for each mandrel, rotary connections for bringing electric current to each heating element and for bringing cooling fluid to each cooling element, and mechanism for actuating the heating and cooling elements in synchronism with turret rotation. Another important disadvantage was that each heating element, by reason of its long period of direct contact with the side wall, could not only melt the coating, as was desired, but could remove areas of coating and the underlying printing.
With the apparatus of this Loeser patent, the bottom seam was intended to be formed with a roller device that was disclosed in the earlier U.S. Pat. No. 2,942,531 to Loeser et al. The roller device of the latter patent merely applied clamping pressure to the bottom blank flange and its adjacent portion of the side wall, relying on glue to form the bond at the bottom seam. The use of glue with thermoplastic-coated blanks was of course inefficient because the thermoplastic coating can serve nicely as an adhesive bonding agent if the machine has satisfactory means for melting the coating on the bottom seam areas of the blanks.
U.S. Pat. No. 3,364,825, issued to D. W. Baumgartner in 1968, disclosed apparatus that represents a more recent state of the art. In the machine of that patent there was a second turret which was rotatable about an upright axis, and it picked up flat side wall blanks and carried each of them edgewise through a pair of heating stations before presenting it to a mandrel on the first turret to be wrapped around it. At each heating station the marginal areas of the side wall blank along its side and bottom edges were subjected to contact heating by means of heating elements that were shaped to conform to the areas to be heated. Two substantially identical heating stations were provided in order to have each heating element in contact with the blank for only a brief period of time. Through such successive brief contacts with heating elements, the seam-forming areas of the coating were brought up to the required temperature for fusion bonding without danger that the blank would be subjected to excessive heat that might cause the coating or any ink on the paper to be picked off, sublimed or charred.
Although the apparatus of the Baumgartner patent was reasonably satisfactory in operation, it had to be maintained in good adjustment. The duration of heating element contact at each station had to be on the order of one-quarter second and was thus critically brief. There was further criticality in that the side wall blank had to be brought into assured contacting engagement with each heating element but excessive clamping force could not be exerted against a blank lest coating be picked off of it and adhere to the element. Furthermore, the temperature of each heating element had to be maintained within predetemined limits, notwithstanding the intermittent withdrawal of heat from the element as blanks moved into and out of heat transfer contact with it, and notwithstanding the tendency of an electric heating element to heat rather slowly.
Since the heating elements that engaged the side wall blank were formed to melt its plastic coating not only along its side edges but also in the zone along the bottom edge of that blank, a bottom seam could be formed by ensuring intimate contact between that zone of the side wall blank and the flange on the bottom blank. To this end the Baumgartner patent recommended employment of the roller device disclosed in the above-mentioned Loeser et al patent. In later practice, however, the bottom seam was bonded by thrusting the bottom blank a distance towards the bottom of the cup to bring it into engagement with molten coating on the side wall blank and, in the case of a tapered cup, to take advantage of the downward taper of the cup side wall.
Although reasonably satisfactor bottom seams could be formed when only the bottom portion of the side wall blank had been heated, it was obviously preferable if the flange portion of the bottom blank were also heated, so that its plastic coating could also contribute to the adhesive bonding at the bottom seam.
In general, it has been evident for some time that contactless heating of the seam forming areas of the blanks is preferable to contact heating of those areas, and it has also been apparent that contactless heating of the blank portions that comprise the bottom seam is both particularly desirable and particularly difficult to achieve with a completely satisfactory heating device.
One prior device for effecting contactless heating of the bottom seam areas has comprised a solid steel bar which was moved cyclically between an open gas flame that maintained it at red heat and a position adjacent to the outer end of the mandrel, within the bottom portion of the side wall blank, where the hot bar effected radiant heating of the seam-forming portions of the blanks. Although this device was satisfactorily effective when operating normally, its need for an open gas flame was undesirable, not only because of the obvious fire hazard but also because the gas flame could be blown out, necessitating shut-down of the machine. Furthermore, the surface of the bottom blank flange on which melting of coating was needed was its radially outer surface, but the coating on that surface was in the "shadow" of the flange itself, and therefore radiant heat could not reach it effectively.
Another expedient for heating the bottom portion of the cup also required a gas flame, but in that case hot gases from the flame were directed towards the cup bottom.
It might be supposed that an electric heating element could be readily substituted for a gas flame in a device for heating the portions of cup blanks that comprise the bottom seam. However, the fact that gas flames continue to be used in cup forming machines, notwithstanding their obviously undesirable characteristics, demonstrates that this seemingly obvious substitution is not, in reality, one that is easily accomplished. An electric heating element tends to heat comparatively slowly, and therefore heat should be abstracted from it at a fairly steady rate in order to ensure that it will maintain a stable temperature.
Another and more significant characteristic of electric heating elements is their inability to withstand vibrations and high accelerations. In a paper cup machine, the device that heats the bottom portion of the cup must move toward the outer end of the mandrel while the mandrel dwells at the station at which such heating takes place, and the heating device must thereafter move back away from the mandrel, in order to be clear of the cup, before the mandrel is carried away from that station by turret rotation. Since paper cup machines produce substantially in excess of 100 cups per minute, it is evident that an electric heating element cannot be allowed to partake of the necessarily rapid advance and withdrawal of the heating device that must take place during the fractional-second dwell of a mandrel at the heating station. Furthermore, if the electric heating element is stationarily mounted and is used to heat a stream of air that is delivered to a nozzle for discharge towards the cup bottom, the air stream passing the heating element cannot be allowed to develop any substantial pulsation as a result of advance and retraction of the nozzle, because such variations in stream flow would impart vibrations to the heating element that would break it in a surprisingly short time.
Since an electric heating element employed for heating the bottom-seam-forming portions of cup blanks must be mounted in a fixed position, it must obviously be remote from the heat applying device that moves toward and from the turret axis, and therefore heat has to be conducted from the heating element to that device. It is apparent that a moving stream of air--provided it has steady flow--affords a satisfactory medium for such heat transfer, and is also a desirable medium for contactless application of heat to the seam forming portions of cup blanks. But because of the heat transfer characteristics of air, not enough heat can be applied to the cup blanks by merely blowing hot air against them during the limited period of dwell available at the bottom heating station under practical conditions of air flow rate and heating element size.
Another consideration that has to be borne in mind with respect to heating devices in a paper cup machine is that the machine can be stopped on occasion--either intentionally or because of a malfunction--while cup blanks are in juxtaposition to the heating devices. It goes without saying that heating devices in commercially available cup machines function satisfactorily under normal conditions of machine operation, but in many cases heat continues to be applied to cup blanks during any stoppage of the machine, and the overheated cup blanks are ruined. Loss of a few cup blanks is in itself inconsequential, but the significant problem that almost invariably arises when cup blanks are overheated is that the overheated material of the cup blank and its coating sticks to the heating devices and other parts of the machine and has to be completely and laboriously removed before the machine can be put back into operation.
Thus a completely satisfactory cup machine should form tight, neat, heat fused bottom and side seams, should have electric elements as its heat sources, and should not need clean-up after a stoppage or shut down. So far as is known, no cup making machine heretofore available has fully met all of these criteria.